The present invention relates to electrochromatic glazings wherein the transmission of light through the glazing may be controlled by changing the coloration of the glazing with an electric field. The electrochromatic glazing of present invention is particularly useful to control the intake of solar heat in buildings or the passenger compartment of automobiles.
Electrochromatic glazings comprise a film of an electrochromatic material that is well suited receive or release cations, which are generally lithium protons or ions. The oxidation state of the cations, corresponding to whether the cations are inserted in or released from the material film, will cause the cations to exhibit different colors. For example, tungsten trioxide will change from a colorless oxidized state to dark blue reduced state according to the chemical reaction: EQU WO.sub.3 +xM.sup.+ +xe.sup.- .rarw..fwdarw.M.sub.x WO.sub.3.
For this reaction to take place, it is necessary to have a source of cations and a source of electrons along the film of electrochromatic material. The source of cations is preferably composed of an electrolyte film having ionic conductivity and the source of electrons is preferably an electrically conductive film. Further, a counterelectrode is also provided which is capable of receiving or releasing cations in conjunction with the film of electrochromatic material. For example, if a cathodic electrochromatic material comprises tungsten oxide, the counterelectrode should preferably comprise an anodic electrochromatic material such as iridium oxide. Iridium oxide is colorless in the reduced state and grey-yellow in the oxidized state, and the insertion of oxygen takes place according to the chemical equation: EQU Mx-I.sub.r O.sub.y .rarw..fwdarw.I.sub.r O.sub.y +XM.sup.+ +xe.sup.-
For this second equation, the cathodic source is an electrolytic film on a glazing, and the electron source is a second electrically conductive film. The two electrically conductive films form two electrodes between which an electrical potential difference is applied.
The electrolyte film, which is a fundamental element of the glazing, must be a good ionic conductor for short switching times and must possess the lowest possible electronic conductivity, i.e. the ionic conductivity divided by the electronic conductivity must be greater than 10.sup.8. The term "switching time" is to be understood as meaning the time period that elapses during passage from a first coloration state to a second colored or decolored state, or the time required for the passage of a quantity of corresponding charges. The quantity of corresponding charges is defined by the integrated current I(t), which is a function that strictly decreases with time.
The electrolyte film must be chemically inert with respect to the other films in the glazing, the electrolyte film must be an acidic or basic liquid electrolyte. However, such a liquid electrolyte presents problems of processing for large-area glazings. Furthermore, the electrolyte film must be transparent so as to permit high light transmission through the glazing system when it is in the colorless state. Finally, the electrolyte film must be prepared with a uniform thickness to avoid short-circuits.
Macromolecular materials having ionic and notably protonic conductivity are especially well-adapted for the production of electrolytic films for large-size electrochromatic glazing systems. This is especially true for solid protonic electrolytes that do not comprise hydrated protons, because hydrated protons reduce the redox stability range of these materials. U.S. Pat. No. 4,844,591 proposes to use a solid solution of anhydrous phosphoric acid in polyoxyethylene as an electrolyte, wherein the O/H ratio of the number of oxygen atoms of the polymer to the number of atoms of the acid equal 0.66:1.
This type of electrolyte is a good protonic conductor at ambient temperatures. For example, a protonic conductivity of 9.times.10.sup.-5 1/3.sup.-1 cm.sup.-1 is obtainable at 20.degree. C. However, this protonic conductivity is increased by a factor of fifty if the temperature increases to 80.degree. C. Although, virtually isothermal operating conditions exist in display applications of electrochromatic glazing systems, these conditions do not exist when these glazings are used in buildings or automobiles because these glazings are exposed to solar radiation. The longer the glazings remain colored, the more solar energy they will absorb, and, in practice, temperatures of 100.degree. C. or more are often measured.
Normally, a high ionic conductivity is preferred to achieve optimum operation of the glazing system, since a higher mobility of the ions provides for shorter switching times. However, this point cannot always be confirmed experimentally because other limiting factors, such as the rate of diffusion of the ions and electrons in the films of electrochromatic materials must also be considered. However, a high ionic conductivity poses a problem because it influences the non-linear relation between the difference in potential across the terminals of the glazing system and the current intensity at the same instant.
From the electrochemical aspect, the difference in potential V.sub.A -V.sub.B applied at a given instant between two points A and B, each belonging to one of the electrically conductive films, may be written as: EQU V.sub.A -V.sub.B =E.sub.A -E.sub.B +.SIGMA..mu.+.SIGMA.RI
where .SIGMA..mu. represents the sum of the electrochemical overvoltages and is essentially a function of the interfaces; .SIGMA.RI is the sum of the potential losses by ohmic drop, which, according to Ohm's Law, are a function of the resistances of the different films of the system, including the resistance of the electrically conductive films and of the electrolyte and the resistance of the current intensity I at this same instant; and E.sub.A -E.sub.B is the electrochemical potential difference between the two points A and B, or, in other words, the potential effectively available for the chemical reactions taking place in the system.
This difference in electrochemical potential E.sub.A -E.sub.B must be sufficiently high, in absolute values, for the insertion and de-insertion reactions of cations in the films of electrochromatic materials to be able to occur. The coloration of the glazing system described above, based upon tungsten and iridium oxides, is thus thermodynamically possible only for a potential difference greater than 0.6 volts In the reverse reaction, starting from the colored state, decoloration does not require the application of a potential difference because the glazing system is similar, in this configuration, to an accumulator while discharging. However, this decoloration may be accelerated if a non-zero absolute value potential difference is applied because the kinetics of the reactions will be faster. Therefore, it is advantageous to operate the glazing system of the present invention with high absolute value potential differences for both the insertion and de-insertion reactions.
However, this high potential difference must be lower than the thermodynamic potentials of other parasitic reactions in the glazing system. These other parasitic reactions include the reduction of protons to molecular hydrogen or the oxidation of traces of water to oxygen. Thus, considering the overvoltages at the interfaces between the layers of film, the upper limit may be set at 1.5 volts for the insertion reaction of protons into tungsten trioxide and at -0.6 volts for the de-insertion reaction, which is, in effect, the insertion reaction into iridium oxide.
Because of these limitations, the problem with an electrochromatic glazing system is that of easily controlling the voltage V.sub.A -V.sub.B while not affecting the value of the electrochemical potential difference E.sub.A -E.sub.B, since the latter is itself a function of the initial current intensity. Without going further into the mathematical considerations relating to these electrochemical equations, if the voltage V.sub.A -V.sub.B remains unchanged, an increase in the temperature of the glazing system will cause a reduction in the resistivity of the electrolyte, an increase in the instantaneous current intensity and a risk of degradation of the glazing system, which is now situated outside its redox stability range. Accordingly, it is desirable to a have an electrochromatic glazing in which the electrical supply determining the voltage V.sub.A -V.sub.B compensates for this temperature factor.